Serial and parallel LED configurations for linear lighting modules

ABSTRACT

A linear lighting module includes a first string of series-connected LED dies and a second string of series-connected LED dies. The first string of LED dies is coupled in parallel with the second string of LED dies. All of the LED dies of the first and second strings are aligned with respect to one another. The LED dies of the first string and the second string form a combined string of interleaved LED dies such that an LED die of the second string is disposed between every successive pair of LED dies of the first string. The LED dies of the combined string are mounted on a flexible substrate. Each LED die of the combined string is electrically connected to two conductors. Except for the two end LED dies of the combined string, each successive LED die must be accessed by both conductors from alternating sides of the combined string.

TECHNICAL FIELD

The present invention relates generally to packaging for light-emittingdiodes, and more particularly to configurations for connecting LEDs inseries and in parallel in linear lighting modules such as tubes.

BACKGROUND INFORMATION

For many years, fluorescent lamps were the most common type of lightfixture used in commercial lighting applications. These traditionallighting fixtures use fluorescent bulbs in recessed troffers withparabolic reflectors. The most common fluorescent bulbs are the linearT5 (⅝ inch diameter), T8 (1 inch diameter), and T12 (1½ inch diameter)bulbs. However, light emitting diodes (LEDs) can more efficientlyconvert electrical energy into light than can fluorescent bulbs. Thus,efforts have been made to incorporate LEDs into tubes having the formfactor of T5, T8 and T12 fluorescent bulbs. LEDs can then be used toretrofit traditional fluorescent light fixtures by installing LED bulbsin the troffers that were originally designed for fluorescent bulbs. Theexisting fluorescent lamp ballasts must then be replaced with new powersupply drivers for the LEDs.

The LED bulbs are typically configured as strings of series-connectedLEDs coupled in parallel. Multiple LEDs are connected in series to formindividual LED strings, and then the LED strings are coupled in parallelto a common voltage and current source. The buck converter of the LEDdriver can operate at a higher efficiency if the wall voltage poweringthe LED bulb is a multiple of the driver's output voltage. Thus, apopular LED driver in North America converts a 120-volt wall voltageinto an output voltage of thirty or forty volts. The band gap ofgallium-nitride LEDs commonly used for lighting is about 3.3 volts. Soto correspond to the output voltage of a popular LED drive, the voltagedrop across a string of series-connected LEDs can be set at about thirtyvolts by connecting nine LEDs in the string. A buck converter can thenefficiently convert the 120-volt wall voltage to the thirty voltsrequired to light the string of nine LEDs.

FIG. 1 (prior art) shows an LED bulb 10 formed by mounting seventy-twopackaged LEDs onto a long substrate anchored in a tube 11 that resemblesa T8 bulb. The transparent curved cover is not shown in FIG. 1. The LEDshave been connected as eight series-connected strings of nine LEDs each.The eight strings are connected in parallel to power and ground. An LEDdriver housed below the long substrate converts the 120-volt wallvoltage to the thirty volts required to light each of the eight stringsof nine LEDs. In the LED bulb 10 shown in FIG. 1, an entireseries-connected string of LEDs is not lit. The third string from theleft is not lit because one of the LEDs in the string is defective orbecause a solder connection in the string shorted out. If the circuit isbroken at any location along a series-connected string, the entirestring goes dark. As LED bulbs are made with larger numbers of LEDs, theprobability that a soldering or LED defect will occur rises much fasterthan merely the proportional increase in the number of LEDs.

Consumers tend to notice a dark portion of an LED bulb caused by ashorted string of LEDs. Even if the amount of light generated by the LEDbulb with the dark portion is nearly as bright as the light produced bya bulb with all of the LEDs lit, the consumer will still perceive thedark portion as a defect and is likely to return the LED bulb. Thus,manufacturers of LED bulbs are faced with higher percentages of returnedLED bulbs as bulbs with larger numbers of LEDs are being sold.

A method is sought for packaging LED dies in tubes resemblingfluorescent bulbs so that inevitable LED and electrical connectiondefects will not result in a return rate that rises faster than theincrease in the number of LEDs in the LED bulbs.

SUMMARY

A linear lighting module includes a first string of series-connected LEDdies and a second string of series-connected LED dies. The first stringof LED dies is coupled in parallel with the second string of LED dies.All of the LED dies of the first string and the second string arealigned with respect to one another. In one embodiment, the strings ofLED dies are arranged in straight lines, whereas in other embodiments,the strings of LED dies are aligned in a curved line. The LED dies ofthe first string and the second string form a combined string ofinterleaved LED dies such that an LED die of the second string isdisposed between every successive pair of LED dies of the first string.The LED dies of the combined string are mounted on a flexible substrateand are disposed in a tube. Each LED die of the combined string iselectrically connected to two conductors. Except for the two end LEDdies of the combined string, each successive LED die must be accessed byboth conductors from alternating sides of the combined string.

In one embodiment, the LED dies of the combined string are mounted ontoa flexible substrate with a single metal layer. The LED dies of thefirst string are electrically connected to one another by a first groupof interconnect tracks formed in the metal layer. The LED dies of thesecond string are electrically connected to one another by a secondgroup of interconnect tracks such that none of the first group ofinterconnect tracks crosses any of the second group of interconnecttracks. Thus, no two of the interconnect tracks are stacked over oneanother and separated by an insulator. In one embodiment, more room ismade available between the LED dies on the substrate by using chip scalepackaged LED dies whose sides are not longer than 1.8 mm.

In another embodiment, a strip of flexible printed circuit board withpre-mounted and wired LED dies is produced in rolls. The strip of LEDshas a first linear array of LED dies connected in series and a secondlinear array of LED dies also connected in series. The first lineararray of LED dies is coupled in parallel with the second linear array ofLED dies. The first linear array of LED dies and the second linear arrayof LED dies are collinear in that all of the LED dies are arranged inone line. However, the LED dies are not necessary arranged in a straightline such as when the strip of LEDs is rolled. The LED dies of the firstlinear array are interleaved between the LED dies of the second lineararray such that no two LED dies of the first linear array areimmediately adjacent to one another without an intervening LED die ofthe second linear array. Each LED die of the combined string ofinterleaved LED dies is electrically connected to two conductors. Exceptfor the end LED dies of the combined string, each successive LED die isaccessed by both conductors from alternating sides of the combinedstring. A segment of a roll of the flexible strip of pre-mounted andwired LED dies is inserted into a tube and used to make an LED bulb.

A method of making an LED bulb that has series-connected strings ofinterleaved LED dies wired using a non-crossing wiring arrangementinvolves connecting the strings in series and in parallel. The LED diesare connected by interconnect tracks formed in a metal layer of aflexible PCB such that none of the interconnect tracks crosses any otherof the interconnect tracks. The LED dies can be mounted to the PCB afterthe interconnect tracks are formed. A first string of LED dies iselectrically connected in series, and a second string of LED dies iselectrically connected in series. The first string of LED dies iscoupled in parallel with the second string of LED dies such that thefirst string of LED dies is aligned with the second string of LED dies.The LED dies of the first string are interleaved with the LED dies ofthe second string such that an LED die of the second string is disposedbetween every successive pair of LED dies of the first string. Couplingthe first string with the second string forms a combined string ofaligned LED dies. Each LED die of the combined string is connected to apower supply conductor. Except for the end LED dies of the combinedstring, the power supply conductor to each successive LED die isconnected from an alternating side of the combined string.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (prior art) is an LED bulb formed by mounting LEDs onto a longsubstrate anchored in a tube that resembles a T8 fluorescent bulb.

FIG. 2 is a diagram of a novel interleaved connection configuration fora linear lighting module that includes linear arrays of interleaved LEDsmounted onto a long PCB that can be placed in a tube.

FIG. 3 shows a configuration of series-connected strings of LEDs thatare coupled in parallel but that are not interleaved among one another.

FIG. 4 is a diagram that compares the locations of the unlit LEDs in theconnection configurations of FIGS. 2 and 3 that would result from adefect somewhere along a series-connected string.

FIG. 5 is a diagram showing two alternative methods of wiring LED dieswith the interleaved connection configuration of FIG. 2.

FIG. 6 illustrates a non-crossing wiring arrangement that connectsinterleaved LED dies of four series-connected strings of ten LED dieseach.

FIG. 7 shows the non-crossing wiring arrangement of FIG. 6 modified sothat the landing pads to which the wire bonds connect to the LED diesare all on the same side of the combined string.

FIG. 8 shows a rolled strip of flexible PCB to which LED dies have beenmounted and wired using the interleaved connection configuration of FIG.2 and the non-crossing wiring arrangement of FIG. 6.

FIG. 9 shows an LED bulb with series-connected strings of LED diesinterleaved according to the connection configuration of FIG. 2 andwired using the non-crossing wiring arrangement of FIG. 5.

FIG. 10 shows an end of the LED bulb of FIG. 9 in more detail.

FIG. 11 is a flowchart of steps for making LED bulb of FIG. 9.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 2 shows a novel interleaved connection configuration 20 for alinear lighting module that includes a linear array of light emittingdiode (LED) dies mounted onto a long printed circuit board 21 that canbe placed in a tube. The interleaved connection configuration 20 isparticularly advantageous for use in LED bulbs in which the linear arrayof LEDs is placed in a tube that resembles a T5, T8 or T12 fluorescentbulb. For example, configuration 20 could be used in a 4-foot T8 bulb.For illustration purposes, FIG. 2 shows a connection configurationlimited to eight series-connected strings of five LEDs each. Otherembodiments of the connection configuration 20, however, have differentnumbers of LEDs. For example, a 4-foot T8 bulb would typically have moreLEDs, such as seventy-two or eighty-eight, which would be configured aseight strings of nine LEDs or eight strings of eleven LEDs,respectively. In addition, the interleaved connection configuration 20of the forty LEDs shown in FIG. 2 could alternatively be implemented asfour strings of ten LEDs.

All forty of the interleaved LED dies shown in FIG. 2 are aligned withrespect to one another. Although the LED dies are arranged in a straightline in FIG. 2, the line of LEDs could also be curved in otherembodiments. For example, the line of LED dies could curve in the xydimension of FIG. 2 if the LEDs were used in a U-shaped tube.Alternatively, the line of LEDs would curve in the z dimension (out ofthe page of FIG. 2) if the LEDs were mounted onto a flexible PCB stripthat is rolled. Thus, the aligned LED dies in configuration 20 need notbe configured in a straight line.

The LED dies of each of the series-connected strings shown in FIG. 2 areinterleaved among the LED dies of the other series-connected strings.For example, the LED dies of a first string are interleaved among theLED dies of a second string in FIG. 2. The five LED dies of the firststring are labeled “1” and are electrically connected by the conductor22 indicated by the dashed line. The LED dies of the second string arelabeled “2”. The first string of LED dies is coupled in parallel withthe second string of LED dies because the input nodes of both stringsare coupled to the power input terminal “+”, and the output nodes ofboth strings are coupled to the ground output terminal “−”. An LED dieof the second string is disposed between every successive pair of LEDdies along the first string. For example, the LED die 23 of the secondstring is disposed between the second successive pair of LED dies 24-25of the first string, and the LED die 27 of the second string is disposedbetween the third successive pair of LED dies 25-26 of the first string.

FIG. 3 shows a configuration of eight strings of five LEDs in which theLEDs of the strings are not interleaved among one another. For example,all of the LEDs of the third string 28 are arranged adjacent to oneanother. If one of the LEDs in the string 28 is defective or burns outor if a solder connection in the string 28 shorts out, then none of thefive LEDs in the third string 28 can be illuminated. A consumer of anLED bulb with the connection configuration of FIG. 3 would surely noticethe dark portion of the bulb potentially caused by a single defectiveLED and would likely return the bulb. However, the same defect in a bulbwith the connection configuration 20 of FIG. 2 would not result in adark portion of the bulb.

FIG. 4 compares the locations of the dark LEDs in the connectionconfigurations of FIGS. 2 and 3 that would result from a defectsomewhere along the third string. The upper row of LEDs in FIG. 4illustrates the positions of the series-connected LED dies of the thirdstring that would remain unlit in the connection configuration 20 ofFIG. 2 with a short in the third string. Each unlit LED die has athicker border. The lower row in FIG. 4 illustrates the positions of theLED dies of the third string that would remain unlit in the connectionconfiguration of FIG. 3 with a short in the third string. Although aconsumer would surely notice a dark portion equaling one eighth of thelength of the bulb as shown in the lower row, the consumer wouldprobably not notice the dark LED dies of the third string in the upperrow that are interspersed among the lit LED dies of the other strings.In addition, where the LED bulb has a diffusing milky cover as opposedto a transparent glass cover, the individual dark LED dies are moredifficult to distinguish. Finally, the lit LED dies would be brighterand would compensate for much of the lost light output of the LED diesin the third string.

The LED drive current is divided between the various strings based onthe total voltage drop across the series-connected LED dies of eachstring. A typical blue LED has a forward voltage of about 3.3 volts,which can vary by as much as a 20%. The variation in the total voltagedrop across the strings causes an imbalance between the drive currentsthat flow through the various strings. Fortunately, the differences inthe forward voltages of multiple LED dies tend to cancel each other out.The probability that the LED dies in one string will all have either ahigher or a lower forward voltage than 3.3 volts decreases as more LEDdies are added to each string. For example, the total voltage dropacross multiple strings of eleven LED dies does not vary significantly.

However, if an LED die in a series-connected string fails open, then noLED drive current will flow through that string, and the current thatwould have flowed through the shorted string is diverted to theremaining strings. The current in each of the other strings willincrease by a factor of 1/(S−1), where S is the total number of strings.Thus, if there is a short in the third string of LED dies in FIG. 2,then the LED drive current will increase by one seventh in each of theother strings as the drive current from the third string is divertedevenly to the other strings.

Except when an LED is “overdriven” by high drive currents, the lightoutput from the LED varies nearly proportionally to the drive current.So the light output of the LEDs in each of the remaining powered stringsin FIG. 4 will increase by almost one seventh, and the total lightoutput by the remaining thirty-five LEDs that are lit will be almost asgreat as the light output by the same drive current flowing through allforty LEDs. A consumer who does not notice any dark spots fromindividual dispersed unlit LEDs in the third string will probably notperceive any reduced light output from the LED bulb and will not returnthe bulb for being defective. At larger LED drive currents, however, anincremental increase in the drive current results in an ever smallerproportionate increase in the luminous flux output by the LED die.Moreover, the operating lifetime of an LED die is significantly reducedwhen the LED die is overdriven. Thus, it is advantageous to use manyseries-connected strings S so that the proportionate increase in the LEDdrive current by 1/(S−1) that results when one string shorts out issmall and does not results in the remaining LED dies being overdriven.For example, when one of eight strings shorts out, the current throughthe remaining strings increases by less than 15%, which is usually notsufficient to overdrive the remaining LED dies. Connecting the LED diesof each series-connected string in an interleaved combined string,however, becomes more complicated as each successive string is added tothe combined string.

FIG. 5 shows two alternative methods of wiring the forty LED dies thathave the interleaved connection configuration 20 of FIG. 2. The topwiring arrangement in FIG. 5 is the method used to connect the LED dieson the printed circuit board 21 of FIG. 2. The bottom wiring arrangementin FIG. 5 is a novel way to connect the forty LED dies using a singleconductive layer that precludes the crossing of any conductors.

The top wiring arrangement in FIG. 5 connects the LED dies usingconductors that cross each other multiple times. For example, theconductor that connects two LED dies of a series-connected string mustcross the seven conductors that connect the LED dies of each of theother seven strings. This wiring arrangement cannot be implemented on asingle conductor PCB, but would most easily be realized on a printedcircuit board with eight conductive layers. The conductors would crossover each other using vias that connect the various conductive layers ofthe PCB. Implementing the connection configuration 20 on a printedcircuit board having a single conductive layer, however, has twoadvantages. First, a PCB with a single metal layer is less expensive toproduce than a PCB with multiple metal layers. Second, a printed circuitboard with a single conductive layer is thinner than a multi-layer boardand therefore can be made more flexible. In the manufacturing process ofmaking LED bulbs, it is desirable to source the LED dies from rolls offlexible strips of printed circuit board with pre-mounted and wired LEDdies. The appropriate length of the combined and interleaved LED stringscan then be unrolled, cut and attached to a flat surface in the LEDbulb. Segments from rolls of pre-mounted LED dies can be inserted intodifferent kinds of housings to make various models of LED bulbs.Therefore, a method is sought that allows multiple strings ofinterleaved LED dies to be connected using a flexible, single-layerprinted circuit board.

The bottom wiring arrangement in FIG. 5 illustrates a method for using asingle-layer conductor to connect the interleaved LED dies of each ofthe eight series-connected strings of five LED dies. The wiringarrangement can be seen in more detail in the expanded portion of thebottom wiring arrangement showing the conductors connected to tensuccessive LED dies along the combined string. No conductor in thebottom wiring arrangement crosses any other conductor. Each LED die ofthe combined string of forty LED dies is electrically connected to twoconductors. For example, the LED die 29 of the eighth string isconnected to the two conductors 30-31, as shown in the expanded portionof the bottom wiring arrangement. Each successive LED die along thecombined string is accessed by both conductors from alternating sides ofthe combined string. For example, both conductors 32-33 access LED die34 of the first series-connected string from one side of the combinedstring, while both conductors 30-31 access the adjacent LED die 29 ofthe eighth string from the other side of the combined string.

Note that the bottom wiring arrangement can connect all of the LED diesin the combined string using a single conductive layer even if the twoend LED dies 35-36 of the combined string are not reached by bothconductors from the opposite side of the combined string from which theadjacent LED dies are reached. Nevertheless, the two end LED dies 35-36in the bottom wiring arrangement of FIG. 5 are accessed by bothconductors from the opposite side of the combined string from which theadjacent LED dies are accessed.

The LED drive current to each of the LED dies is received through one ofthe conductors and passes towards the ground terminal through the otherconductor. For example, LED die 29 of the eighth string is connected toa power supply conductor 31 and to a conductor 30 that passes thecurrent towards ground. Each successive LED die along the combinedstring is electrically connected to a power supply conductor thatconnects to the die from an alternating side of the combined string. Forexample, the power supply conductor 33 connects to the LED die 34 of thefirst series-connected string from one side of the combined string,while the power supply conductor 31 connects to the adjacent LED die 29of the eighth string from the other side of the combined string.

FIG. 6 illustrates another embodiment of the non-crossing wiringarrangement that connects interleaved LED dies of four series-connectedstrings of ten LED dies each. FIG. 6 shows a first linear array of tenLED dies labeled with is connected in series and a second linear arrayof ten LED dies labeled with 2s connected in series. There are alsothird and fourth linear arrays in the combined string of interleaved LEDdies. The first and second linear arrays are coupled in parallel and arecollinear. The LED dies of the first linear array are interleavedbetween the LED dies of the second linear array. No two LED dies of thefirst linear array are immediately adjacent to one another without anintervening LED die of the second linear array. For example, an LED dieof the second string labeled with a “2” is disposed between eachsuccessive pair of LED dies of the first string labeled with 1s. EachLED die is connected to a first conductor that is coupled to the powerinput terminal “+” and to a second conductor that is coupled to theground terminal “−”. Except for at the ends of the strings, theconductors are coupled to power and to ground through the other LED diesof each series-connected string. And except for the end LED dies 37-38of the combined string, each successive LED die is accessed by both thepower and ground conductors from alternating sides of the combinedstring. Thus, the power supply conductor to each successive LED dieconnects from an alternating side of the combined string. For example,the LED die 39 of the first linear array is connected to both a powerconductor 40 and a ground conductor 41 from one side of the combinedstring, while the next LED die 42 along the combined string, whichbelongs to the second linear array, is connected to both a powerconductor 43 and a ground conductor 44 from the other side of thecombined string.

FIG. 7 shows another embodiment of a non-crossing wiring arrangementthat connects interleaved LED dies of four series-connected strings often LED dies each. The wiring arrangement of FIG. 7 differs from that ofFIG. 6 in that the landing pads to which the wire bonds connect to theforty LED dies in FIG. 7 are all on the same side of the LED dies. InFIG. 6, the wire bonds to each successive LED die connect to landingpads on alternating sides of the LED dies. Nevertheless, except for theend LED dies 37-38 of the combined string, each successive LED die inFIG. 7 is still accessed by both the power and ground conductors fromalternating sides of the combined string despite the fact that theconductors contact all of the LED dies on the same side. The conductorsaccess an LED die from the side from which the conductors emerge to wraparound the adjacent LED dies. For example, LED die 42 of the secondlinear array is attached to both the power conductor 43 and the groundconductor 44 on the upper side in FIG. 7, but LED die 42 is neverthelessaccessed from and connected to both conductors 43-44 from the lower sideof the combined string in FIG. 7. The power conductor 43 passes aroundthe adjacent LED die of the third linear array from the lower side, andthe ground conductor 44 passes around the adjacent LED die 39 of thefirst linear array from the lower side. In an alternative embodiment,the LED dies are not connected to the conductors by wire bonds. Rather,the LED dies are flip chip mounted and contact the ends of theconductors directly.

FIG. 8 shows an unraveling roll 45 of LED dies wired and mounted to astrip of flexible printed circuit board 46. The LED dies on the roll offlexible PCB 46 are connected using the interleaved connectionconfiguration 20 of FIG. 2. In addition, the interleaved LED dies arewired using the non-crossing wiring arrangement of FIG. 6. Theconductors that electrically connect the LED dies to each other and topower and ground are formed from a single conductive layer of theflexible PCB 46. Some of the LED dies on the roll 45 of FIG. 8 have beenlabeled to indicate that they belong to one of four series-connectedstrings of ten LED dies each. An LED bulb can be manufactured using alength of the strip of mounted LED dies. For example, a strip of theflexible PCB 46 somewhat less than four feet long can be attached withthermal glue to an aluminum substrate that fits inside a tube in orderto make a 4-foot T8 LED bulb. To simplify the drawings, the non-crossingwiring arrangement of FIG. 6 is illustrated with four series-connectedstrings of ten LED dies each. However, an LED bulb would just as likelyhave more total LEDs. For example, a more likely configuration would beeight strings of eleven LEDs.

FIG. 9 shows a 4-foot T8 LED bulb 47 that has eighty-eight LED diesconfigured as eight series-connected strings of eleven LEDs each. TheLED dies of the eight strings are interleaved according to theconnection configuration 20 of FIG. 2 and are wired using thenon-crossing wiring arrangement of FIG. 5. Each string of eleven LEDdies has a total voltage drop of about 37.4 volts where thegallium-nitride LED dies have an average individual band gap of 3.4volts. The eight strings are driven by a buck converter that convertsthe 120-volt alternating wall current to a direct current with about 40volts. The LED driver is, however, a constant current driver as opposedto a constant voltage driver. FIG. 9 illustrates the case in which anLED die of the third series-connected string of eleven dies 48-58 hasshorted out so that all of the dies in the string are dark. Because theseries-connected strings of LED dies have been interleaved, the elevenunlit LED dies 48-58 are hardly noticeable. When current no longer flowsthrough the shorted third string, the current flowing through each ofthe other seven strings increases by one seventh so that the lightoutput by the remaining seventy-seven LED dies increases by almost oneseventh to compensate for the lost light output from the third string.

FIG. 10 shows an end of the LED bulb 47 of FIG. 9 in more detail. Thetube of bulb 47 is made in two sections. A top section 59 of the tubeslides into grooves in the bottom section 60. In this embodiment, thetop section 59 is translucent plastic that diffuses the light emittedfrom the LED dies. No black spots are apparent in the light that shinesthrough the top section 59 of the tube even when a shorted string of LEDdies is not lit. In this embodiment, the bottom section 60 of the tubeis made of extruded aluminum. The bottom section 60 has a curved portionand a flat portion. The flat portion has the grooves into which the topsection 59 snaps or slides. The strip of the flexible PCB 46 withmounted and wired LED dies is attached to the flat portion. FIG. 10shows that the LED dies on the strip are connected by a single layer ofnon-crossing interconnect tracks on the flexible substrate 46.

In this embodiment, the LED dies are chip scale packaged LED dies inwhich the semiconductor die is mounted directly to the substrate 46without any additional intervening substrate. For example, each LED dieis a volumetric white chip in which silicone with embedded phosphorparticles conforms directly to the chip. This allows the size of theindividual LED dies to be smaller, which leaves more room for theinterconnect tracks. In this case, the LED dies are rectangular with noside longer than 1.8 mm. With S strings of series-connected LED dies,S-2 interconnect tracks must fit between the adjacent LED dies of thefirst and second strings. Thus, six interconnect tracks must fit betweenthe first and second and between the ninth and tenth LED dies shown onthe flexible substrate 46 of FIG. 10. The smaller chip scale packagedLED dies allow a large number of dies to be mounted in the LED tube withsufficient distance remaining between the dies to accommodate thenon-crossing wiring arrangement of FIG. 5.

FIG. 11 is a flowchart illustrating steps 61-66 of a method of makingthe LED bulb 47 that has series-connected strings of LED dies wiredusing the non-crossing wiring arrangement of FIG. 5 and interleavedaccording to the connection configuration of FIG. 2. In a first step 61,a first string of LED dies is electrically connected in series. Forexample, a first string of eleven LED dies mounted on flexible substrate46 is connected in series by interconnect tracks etched in the top metallayer of the flexible PCB. In step 62, a second string of LED dies iselectrically connected in series on the substrate 46. In step 63, thefirst string of LED dies is coupled in parallel with the second stringof LED dies such that the first string of LED dies is aligned with thesecond string of LED dies. The LED dies are aligned one behind the otheralong the flexible PCB strip. The LED dies need not be aligned in astraight line. For example, the line of LED dies curves when theflexible PCB strip is wound in a roll 45. The series-connected stringsare coupled in parallel by connecting the ends of the strings to thesame power and ground terminals.

In step 64, the LED dies of the first string are interleaved with theLED dies of the second string such that an LED die of the second stringis disposed between every successive pair of LED dies of the firststring. In addition, as is apparent from FIG. 5, every successive pairof LED dies of every string is separated by an LED die of each remainingstring. For example, between every two closest LED dies of the secondstring lies an LED die of each of strings 1, 3, 4, 5, 6, 7 and 8. Inorder to use the interconnect tracks of a single conductive layer offlexible PCB 46 to connect the strings in series in steps 61-62, tocouple the strings in parallel in step 63, and to interleave the LEDdies of the various strings in step 64, each successive LED die alongthe combined string (except for the end LED dies) must be accessed bythe interconnect tracks from alternating sides of the combined string.Thus, the interconnect track that supplies LED drive current to eachsuccessive LED die wraps around the adjacent LED die from an alternatingside of the combined string.

In step 65, the LED dies of the first string and the second string aremounted to the flexible substrate 46. Although the LED dies could beconnected and wired, for example, partly through wire bonds, after thedies are mounted to the substrate, it is also possible to form theelectrical connections entirely as interconnect tracks on the substrate46 before the LED dies or chip-scale-packaged LED dies are flip-chipmounted onto the pre-wired substrate. Thus, in one embodiment of themethod of FIG. 11, the positions of the LED dies are series-connected instrings in steps 61-62, the strings of the LED positions are coupled inparallel in step 63, and the positions of the LED dies in the variousstrings are interleaved in step 64 all before the actual physical LEDdies are mounted onto the substrate 46.

In step 66, the first string of LED dies and the second string of LEDdies are inserted into a tube 59-60. The flexible PCB strip with mountedand wired LED dies is attached with an adhesive to a flat metal surface.In one implementation, the flat metal surface is the flat portion of thebottom section 60 of a tube that also has a curved portion. The topsection 59 of the tube is a transparent or translucent cover thatattaches to the bottom section 60 to form a cylindrical bulb. In anotherimplementation, the long, flat piece of metal to which the flexible PCBstrip with mounted and wired LED dies has been attached is inserted intoa cylindrical glass or plastic tube. The power and ground terminals ofthe LED strip are then connected to the LED driver, which in turn isconnected to the end plugs of the LED bulb, which fit into the socketsof the troffer.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A device comprising: a first string ofseries-connected light emitting diode (LED) dies; and a second string ofseries-connected LED dies, wherein there are S strings ofseries-connected LED dies, wherein S is greater than or equal to eight,wherein the first string of LED dies is coupled in parallel with thesecond string of LED dies, wherein all of the LED dies of the firststring and the second string are aligned with respect to one another,wherein an LED die of the second string is disposed between everysuccessive pair of LED dies of the first string, wherein each of the LEDdies is electrically connected to two conductors, and wherein S-2conductors pass between at least two adjacent LED dies.
 2. The device ofclaim 1, wherein all of the LED dies of the first string of LED dies arearranged along a straight line, and wherein all of the LED dies of thesecond string of LED dies are arranged along the straight line.
 3. Thedevice of claim 1, wherein the LED dies of the first string and thesecond string form a combined string of interleaved LED dies, andwherein except for the two end LED dies each successive LED die isaccessed by both conductors from alternating sides of the combinedstring.
 4. The device of claim 1, wherein the LED dies of the firststring and the second string form a combined string of interleaved LEDdies, wherein each LED die of the combined string is electricallyconnected to a power supply conductor, and wherein except for the endLED dies of the combined string, the power supply conductor to eachsuccessive LED die connects from an alternating side of the combinedstring.
 5. The device of claim 1, wherein the LED dies of the firststring and the second string are mounted on a flexible substrate.
 6. Thedevice of claim 1, wherein the first string of LED dies and the secondstring of LED dies are disposed in a tube.
 7. The device of claim 1,further comprising: a flexible substrate with a metal layer, whereininterconnect tracks are formed in the metal layer, wherein theconductors that electrically connect the LED dies of the first stringare formed by a first group of interconnect tracks, wherein theconductors that electrically connect the LED dies of the second stringare formed by a second group of interconnect tracks, and wherein none ofthe first group of interconnect tracks crosses any of the second groupof interconnect tracks.
 8. The device of claim 1, wherein the LED diesare chip scale packaged LED dies.
 9. The device of claim 1, wherein theLED dies are rectangular with no side longer than 1.8 mm.
 10. The deviceof claim 1, wherein the LED dies are chip scale packaged LED dies whosesides are not longer than 1.8 mm.
 11. The device of claim 1, wherein noconductor crosses any other conductor.
 12. A device comprising: a firstlinear array of light emitting diode (LED) dies connected in series; asecond linear array of LED dies connected in series, wherein the firstlinear array of LED dies is coupled in parallel with the second lineararray of LED dies, wherein all of the LED dies of both the first lineararray of LED dies and the second linear array of LED dies are collinear,and wherein the LED dies of the first linear array are interleavedbetween the LED dies of the second linear array; and an Nth linear arrayof LED dies connected in series, wherein N is eight or greater, andwherein each of the LED dies of each of the N arrays of LED dies iselectrically connected to two conductors, and wherein N-2 conductorspass between at least two adjacent LED dies.
 13. The device of claim 12,wherein none of the conductors crosses any other of the conductors. 14.The device of claim 12, wherein the LED dies of the first linear arrayand the second linear array are mounted on a flexible substrate.
 15. Thedevice of claim 12, wherein the first linear array of LED dies and thesecond linear array of LED dies are disposed in a tube.